1. Field of the Invention
The present invention relates to a compact auto focus lens module with a piezoelectric actuator, especially to an auto focus lens module in which movement of an optical lens set is controlled by displacement and friction produced by deformation of the piezoelectric actuator for focusing.
2. Brief Discussion of the Related Art
Digital cameras or mobile phone cameras include a lens module that drives optical lens set moving along an optical axis so as to achieve autofocusing and/or zoom in/zoom out. One of the movement ways of the optical lens set is by Voice coil motor (VCM). After a current is applied to a coil, a driving force is generated in an electromagnetic field created by a permanent magnet so as to drive the optical lens set moving along the optical axis. Such design can reduce volume of lens modules so that it has been applied to mini-sized cameras, camera-equipped cellular phone or web-cameras broadly, as shown in US2008/0013196, U.S. Pat. No. 6,594,450, U.S. Pat. No. 7,145,738, TW M317027, JP3124292, JP3132575 and so on.
Ultrasonic motor (USM) is a type of electric motor formed from the ultrasonic vibration of piezoelectric material that is deformed while being applied with voltage. Then, by design of a friction drive, the ultrasonic motor moves in a rotational or straight way, as prior arts disclosed in US2009/0153987, US2008/0297923, US2008/0174889 etc. Reference is made to FIG. 1 in which, as revealed in US2008/0246353 etc., the ultrasonic motor is applied to auto focus lens module. A piezoelectric actuator 902 is disposed on the fixed element 9091 and is connected with a lens moving element 9092 by a connector 9021 for generating a piezoelectric driving force. Moreover, a coil element 907, together with a magnet element 908, generates an electric-magnetic force. By a balance between the piezoelectric driving force and the electric-magnetic force, the lens moving element 9092 and the lens set 901 are driven to move along shafts 904 for focusing.
Referring to FIG. 2 in which, as revealed in U.S. Pat. No. 7,480,109, US2008/0085110, etc., a piezoelectric drive optical lens module includes a guiding rod 904 having a frictional surface 9022, and a piezoelectric actuator 902, mounted in an elastic element 903 so as to drive the lens set 9012 as well as the lens barrel 9011 to move along a guide pin 9043 by a guide slot 9042 for focusing.
Referring to FIG. 3, another USM-type auto focus lens module revealed in US2008/0231970 includes a plate spring 903. By an elastic force from the plate spring 903 against two piezoelectric actuators 902, a friction force is generated between a guide rail 904 and a lens barrel 901. However, because the plate spring has complicated structure, high volume requirement and large space, it is not suitable for mini-sized lens modules.
In order to make the lens module move stably without tilting, devices disclosed in US2008/0144201, JP2004020935, etc., include a guide rod arranged between the lens barrel and a housing of the lens module so as to stabilize the lens barrel. In US2007/0153404, a rail is used. Referring to TW 1265357, two sets of piezoelectric actuators are arranged on edges of the lens barrel so as to make the movement thereof become stable. However, conventional ultrasonic motors for movement of the lens barrel have shortcomings of complicated structure, high cost or large volume in designs of the guiding rod or friction generation so that they are difficult to be applied to compact auto focus lens modules. Thus, there is a need to develop a driving member with highly reliable and highly stable structure that replaces conventional ways of generating friction. Moreover, requirements of simple structure, high precision, fast focus and reduced tilting need to be satisfied.